It is important in the design and planning of buildings, particularly offices and homes, to give consideration to prevention of the spread of fire. There are basically three types of fire resistant glass product which are currently used in buildings. Firstly, there is glass which is produced with a wire mesh or other fire resistant material cast or laminated into the structure which of course reduces the visibility through the glass. Whilst this type of glass easily breaks when subjected to heat and flames, the fire resistant material (eg. wire mesh) therein will hold the glass in place to resist spread of fire. This first type of glass product also has the disadvantage of permitting radiation of heat to the area beyond the glass. Secondly, there is clear glass which will not break for at least half-an-hour when subjected to intense heat in a test defined in British Standard BS 476, parts 20, 22 and others, i.e. upon build-up of heat to 900.degree. C. This product will prevent the spread of flame but will not stop radiation of heat to the area beyond the glass. Thirdly, there is glass which will not break for at least half-an-hour when subjected to intense heat (as laid down by BS 476), and will also prevent the radiation of intense heat to the area beyond the glass. This product is currently produced in a series of laminates of glass and other materials which combine to prevent the conduction of heat.
The third type of glass (the laminated product) is generally expensive and heavy, and whilst the first type of glass is considerably cheaper, it is not usually aesthetically pleasing. The second type of glass can give good fire resistance without necessarily being very expensive, and whilst still being aesthetically pleasing.
There are two fire resistant products currently available which are of this second type. The first of these is a sheet of borosilicate glass. Borosilicate glass is of its nature relatively fire resistant. However, it cannot be satisfactorily toughened by tempering and thus does not meet certain "safety glass" requirements such as those of British Standard 6206A. Furthermore, borosilicate glass is more expensive than ordinary silica glass. The second product is a clear ceramic based product which has excellent fire resistance but has very poor optical quality and is not suitable as a simple replacement for ordinary window or panel glass.
It would be highly advantageous to be able to provide a fire resistant glass product of the second type described above, made from ordinary silica based glass since this is relatively inexpensive and it has excellent optical properties. However, silica based glass is not of itself fire resistant: it shatters when exposed to intense heat. Further, toughening the glass by tempering does not significantly affect its fire resistant properties: it still shatters when exposed to intense heat.
We have now found that a fire resistant product of the second type described above, can be made from silica based glass. Thus, we have devised a glass for use in buildings particularly where it is deemed desirable or necessary to prevent the rapid spread of fire and, at the same time, to give the appearance of a standard piece of "coated" glass. In particular, we have found a way in which such a fire-resistant glass can be made in a relatively simple and economic way.